Messages in this thread | | | Date | Tue, 20 Apr 2010 12:35:19 +0300 | From | Avi Kivity <> | Subject | Re: [PATCH 1/5] Add a global synchronization point for pvclock |
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On 04/20/2010 04:57 AM, Marcelo Tosatti wrote: > >> Marcelo can probably confirm it, but he has a nehalem with an appearently >> very good tsc source. Even this machine warps. >> >> It stops warping if we only write pvclock data structure once and forget it, >> (which only updated tsc_timestamp once), according to him. >> > Yes. So its not as if the guest visible TSCs go out of sync (they don't > on this machine Glauber mentioned, or even on a multi-core Core 2 Duo), > but the delta calculation is very hard (if not impossible) to get right. > > The timewarps i've seen were in the 0-200ns range, and very rare (once > every 10 minutes or so). >
Might be due to NMIs or SMIs interrupting the rdtsc(); ktime_get() operation which establishes the timeline. We could limit it by having a loop doing rdtsc(); ktime_get(); rdtsc(); and checking for some bound, but it isn't worthwhile (and will break nested virtualization for sure). Better to have the option to calibrate kvmclock just once on machines with X86_FEATURE_NONSTOP_TRULY_RELIABLE _TSC_HONESTLY.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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